(Ganapathi atharva seersha is a part of Atharvana Veda. It is classified as an Upanishad and not as a chant. But this is considered universally as the greatest and most effective prayer addressed to Ganesa .I have used a small book let giving meaning of this great prayer published by Om graphics and also write up and meaning given in Audarya fellowship and Mathra on line. This is also called Gana Upanishad or Ganapthi Seersha Upanishad,)

My Salutations to the Lord of Ganas,
You are the direct experience of you as me,
You are only the doer of all works, (creator)
You are only the bearer of all burdens, (one who looks after)
You are only the one who destroys(destroyer)

Thwameva sarvam khalvidam brahmasi,
Thwam sakshath athmasi nithyam.

You are the supreme God who conquers,
And you are the real imperishable soul.

You are that which makes me speak.
You are filled with joy, filled with Brahmam.
You are the one without second truth , mind and bliss.
You are the Brahmam which is directly experienced.
You are filled with wisdom and filled with science.

All the universe originates from you,
All the universe exists because of you.
All the universe finally mixes within you.
All the universe is nothing but you.
You are the earth, water fire, air and the sky,
You are the four types of speech and all the words.

You are beyond Sathva, Rajas and Thamas,
You are beyond three types of experiences-waking , sleep and dream,
You are beyond the body states of Gross, subtle and casual,
You are beyond the three divisions of time-past, present and future,
You always and daily stay in mooladhara Chakra,
You are the three states of energy-creation, maintenance and destruction,
You are being daily meditated upon by sages,
Yo are Brahma , you are Vishnu , you are Shiva.
You are Indra, You are fire , you are wind.
You are the Sun, You are the moon,
You are all inclusive and all pervading.

Ga is the first part , Na is the second part,
After that Um followed by a crescent dot ,
Followed by the Om is the full form of the chant,
That is Ga first Aa in the middle,
This is followed by anu and ends with a dot,
When these sounds are merged you get the Manthra,
Ganaka is the sage for this knowledge of Ganesha,
The metere is Nischad Gayathri and god addressed is Ganapathi,
And the chant is “Om Gam danapathaye Nama.”

This is called the head of the Atharva veda,
It makes you capable of merging with God,
It removes all road blocks on your way,
It leads to permanent happiness, and pleasures,
It helps you out of the five great sins.

Meditating it in the evening, removes sins committed in day time,
Meditating in the morning , removes the sins committed at night,
Meditating during evening and morning removes all sins committed,
Meditating on it always does never have any problems.
And you would get Dharma, wealth and salvation.

He who gives ceremonial bath chanting this would become a master of words,
He who reads it with empty stomach on the fourth phase of moon becomes knowledgeable.
This which has been written in Atharvana Veda,
Makes gods like Brahma very dear to you,
And he who chants it becomes one with knowledge,

He who worships with dhurva grass chanting this would become very rich,
He who worships with puffed rice chanting this , would become very famous,
He who worships with thousand modhakas would get all his desires fulfilled,
He who worships with fire sacrifice would get all that he wants.

He who chants with eight Brahmins would get the effulgence of Sun God,
He who chants it during solar eclipse in the shores of great rivers or in front of Ganesa every month,would become the master of this manthra and all he tells would become true,
He would get rid of major road blocks,
He would get rid of great problems,
He would get rid of great dangers,
He would become greatly knowledgable, greatly knowledgable
This is the Veda and this is the Upanishad.